This invention relates generally to the electrofusion welding of profile wall pipes made from thermoplastic materials including polyethylene, polyvinyl chloride, nylons, polybutylene, polypropylene, and the like. Specifically, this invention relates to electrofusion welding of such pipes to obtain a joint which has substantially flush interior and exterior surfaces and provides constant inside and outside diameters, without the need for a coupler.
This invention pertains to the joining of profile wall thermoplastic pipes including polyolefin, polyethylene, polyvinyl chloride, nylon, polybutylene, polypropylene, and the like. These types of pipes are gaining popularity in water, sewer, culverts, and industrial piping because of their characteristics of being lightweight, corrosion resistance, strong, and durable.
xe2x80x9cTrenchlessxe2x80x9d rehabilitation of culverts, storm sewers, sanitary sewers, and other underground pipes by xe2x80x9cslip liningxe2x80x9d or xe2x80x9cinsert renewalxe2x80x9d using thermoplastic pipes is gaining popularity and growing rapidly throughout the United States and other countries. In this process, a thermoplastic pipe or liner is inserted into an existing pipe or culvert without removal of the deteriorated pipe. The replacement pipe is pushed into or pulled through the existing culvert. In many cases, an existing pipeline can be rehabilitated for a fraction of the cost of replacement and with minimal inconvenience to the public.
Thermoplastic pipes, including polyethylene, are the preferred pipe material for many rehabilitation projects because of the price and the above-noted characteristics of such pipe. Generally, thermoplastic pipe is manufactured in lengths which are sufficiently short to permit transportation and handling. In the field where the pipe is to be installed, the short pipe sections must be connected to form a continuous pipe of a predetermined length appropriate for the application.
The joining or connecting of thermoplastic pipes can present many problems because of the variety of field conditions encountered and because of the chemical resistance of the thermoplastic pipes which, in many cases, makes such pipes impervious to glues or cements. Additionally, some thermoplastic pipes have a tendency to xe2x80x9ccreep,xe2x80x9d or move, when subjected to changing temperatures. Because most applications include exposure to such temperature changes, such movement or xe2x80x9ccreepingxe2x80x9d limits the ability to use mechanical type joints such as threads.
In general, several methods exist to join thermoplastic pipe in the field. A first method, known in the art as xe2x80x9cbutt fusion,xe2x80x9d involves the use of a heat fusion machine which includes line up equipment and a heat plate. The ends of two pipes to be joined are inserted into the line up equipment which aligns and advances the pipe ends toward one another as necessary. The two pipe ends are pressed against the heat plate which heats and softens the two pipe ends. The heat plate is then removed and the line up equipment advances the two pipes toward one another at a predetermined rate (depending on the size and thickness of the pipe walls) in order to fuse the pipe ends together. This type of butt fusion requires special fusion equipment that is expensive, is not always available in the field, and cannot be used with certain types of pipe.
Another method known in the art is the use of electrofusion collars or inserts. One type of electrofusion collar is shown in U.S. Pat. No. 4,530,521 to Nyffeler, et al. and one type of electrofusion insert is shown in U.S. Pat. No. 3,768,841 to Byrne at al. These devices, as shown in the references, use a sleeve, collar, or insert made of thermoplastic material which either fits over or into the two pipes being joined. The pipes, and the collar or insert are first heated to soften the thermoplastic material. If using a collar, the pipe ends are inserted into the collar and are thereby joined. If using an insert, the insert is inserted into each pipe end, thereby joining the pipes. The heating can be performed with fusion equipment or, alternatively the collar or insert can contain an electrical resistance element to provide the necessary heat to cause electrofusion welding between the pipes and the collar or insert.
These devices have various disadvantages, including the creation of interior obstructions or exterior protrusions which are not acceptable in many applications. For example, because the collar must be large enough to accept insertion of the pipe ends, the resulting joint does not have a flush exterior. Additionally, because the insert reduces the inside diameter of the pipes at the joint, the insert acts as an obstruction to flow through the pipe. This is unacceptable in most applications, including most trenchless rehabilitation projects, because interior flow obstruction is not acceptable. Furthermore, exterior collars impede insertion of the replacement pipe during slip lining, as known in the art, thereby requiring the use of smaller diameter replacement pipes so that the collar can fit over the pipe and the entire coupling can still fit inside the existing pipe or culvert.
Another method of joining thermoplastic pipes utilizes electrofusion rods or mesh as shown in U.S. Pat. No. 5,410,131 to Brunet et al. Although this method requires no collar or insert, the application requires substantial end pressure to join the two pipe ends. Due to the weight of the pipes, such pressure is usually supplied by special line up equipment and this equipment is expensive and not always available or practical for use in field conditions.
Another method, hot air gun welding, uses a welding rod of thermoplastic material fed through the nozzle of a hot air gun. The hot air gun applies heat to the ends of the pipes being welded and melts the welding rod which is applied to a bevel cut between the two ends of the pipes to be joined. As with the butt fusion methods, line up equipment must be used and this method has not proven satisfactory in field conditions due to a lack of uniformity in the weld.
As thermoplastic resin prices increase, plastic pipe manufacturers are constantly looking for manufacturing methods to make pipe lighter without reducing physical strength. One type of thermoplastic pipe that has been developed to address these concerns, and that is gaining popularity, is xe2x80x9cprofile wall pipexe2x80x9d as it is known in the art. An example of this type of pipe is shown in U.S. Pat. 5,362,114 to Levingston. Profile wall pipe is thermoplastic pipe formed by extrusion to have an inner cylindrical wall, a generally concentric outer cylindrical wall, and a helical rib between and connecting the inner wall and outer wall.
Profile wall pipe is lighter than solid pipe and is created with less material, thereby reducing resin costs, but maintains a high degree of strength. Because of its light weight, profile wall pipe generally has a competitive advantage over solid wall plastic pipe. For these and other reasons, profile wall pipe is popular in the industry. Unfortunately, conventional methods of fusing solid wall thermoplastic pipe are unacceptable and will not work on profile wall pipe. This is due, in part, to the configuration of the end wall of the profile wall pipe which does not provide a solid annular surface due to the presence of the helical rib.
For example, butt fusion is very difficult on profile wall pipe because the pipe ends of profile wall pipe are not solid. The profile wall pipe ends have a thin inner wall, a thin outer wall, and a xe2x80x9cprofile spacexe2x80x9d between the inner and outer walls, the axial depth of which is equal to the distance between the end of the pipe and the helical rib which connects the inner and outer walls. The same problems that exist in joining solid wall thermoplastic pipes are multiplied in profile wall pipes because of their relatively thin inner and outer walls and large profile space between the walls.
For example, one manufacturer produces a polyethylene 10xe2x80x3 inside diameter profile wall pipe that has inner and outer wall thicknesses of 0.065xe2x80x3 to 0.079xe2x80x3. This particular pipe has a profile space of approximately 0.5xe2x80x3 between the inner and outer walls with an outside diameter of up to approximately 11.20xe2x80x3. A 36xe2x80x3 inside diameter profile wall pipe from the same manufacturer has inner and outer wall thicknesses of approximately 0.195xe2x80x3 to 0.228xe2x80x3 and an outside diameter of approximately 40.65xe2x80x3, thereby having a profile space between the inner and outer walls of approximately 2xe2x80x3.
Butt fusion of profile wall pipes is very difficult due to the thin wall thickness compared to the overall diameter. Setting the correct hydraulic pressure on a butt fusion machine for such thin walls and large diameters would result in extremely slow fusion machine carriage movement and potential cooling of the thermoplastic material prior to fusion joining. This results in a failed weld or xe2x80x9ccold jointxe2x80x9d as known in the art. Additionally, rods or mesh will not work on profile wall pipe because profile wall pipe does not have solid flat pipe end surfaces which are required when using those methods.
For example, U.S. Pat. No. 5,494,318 to Butts et al. discloses a secondary containment piping system composed of a plurality of modules of concentric pipe. However, the invention of Butts would not work with profile wall pipe because when joining concentric pipes together using the apparatus of Butts, one must use line-up equipment. Dual containment pipe, unlike profile wall pipe is essentially two separate thermoplastic solid wall pipes with walls of sufficient thickness such that butt fusion is a readily acceptable means of forming joints. In addition, as disclosed in Butts, a welding rod is placed between the solid ends of the pipe members being joined, pressure is applied and maintained while an electric current is passed through wires causing the melting of the core of the welding rod and the adjacent portions of the pipe members. The pressure is maintained after the current is discontinued until the members are fused together. Column 1, lines 35-45. Specifically, Butts discloses positioning an annulus of welding rod between the ends to be joined and butting the ends together with the appropriate maintenance of pressure while an electric current is supplied for a sufficient time to cause fusion of the members and the welding rod. This will simply not work with profile wall pipe for the reasons stated above. Additionally, in order for this type of pipe fusion to work, the wall thickness of the inner and outer walls would have to be increased to such an extent that the advantages of using profile wall pipe would be lost. Moreover, dual containment pipe is not satisfactory for all applications in which profile wall pipe is commonly used for the reasons noted above.
Butts also discloses the use of a xe2x80x9cfusion ringxe2x80x9d for coupling sections of dual containment pipe. However, Butts requires the use of a welding rod element internally within the fusion ring. The ends of the pipe being joined are received in opposite sides of the ring where they are held in position as the welding rod is heated and fused with the ends of the pipe to form the complete joint. The fusion ring is in the form of a collar surrounding the welding rod, the collar having an inner diameter substantially equal to the outside diameter of the sections to be joined. The fusion ring is positioned between two sections of pipe as in a socket fitting and the two sections are then welded together while pressure is applied to push the pipes together.
The fusion ring disclosed in Butts et al. is not acceptable for profile wall pipe and causes interior and exterior protrusions which impede flow and impede the use of the pipe during sliplining of culvert systems. Moreover, dual containment pipe is distinct from, and does not have the advantages of, profile wall pipe. Dual containment pipe is essentially two solid wall pipes concentric with one another. Thus, those methods of joining pipe that work with solid wall pipe will work with dual containment pipe, but not with profile wall pipe. Therefore, the invention disclosed in Butts would not be applicable to nor functional with profile wall pipe to obtain the advantages provided by the invention disclosed herein.
For example, profile wall pipe, unlike dual containment pipe, is not two separate concentric pipes. In fact, profile wall pipe is a single pipe, which, as explained above, is extruded and used for its lightweight replacement capabilities. However, the inner and outer walls of the profile wall pipe are so thin that the welding rod of Butts would not work to butt weld profile wall pipe and, for the reasons discussed above, nor can one easily butt fuse profile wall pipe using line-up equipment due to the complexity of the process and the length of time and pressure that would be required to cause the pieces of pipe to fuse together. Moreover, in order to prevent the protrusion of the welding rod into and out of the pipe so as to maintain flush surfaces at a butt weld, one would be required to use such a thin welding rod that it would not be sufficient to carry enough current and make a sufficiently consistent weld.
Another method, hot gas welding, when used with profile wall pipes, does not achieve a strong, uniform joint even with the most experienced welders. Prior art electrofusion collars or inserts are very bulky and either seriously interrupt interior fluid flow or have a very large collar on the outside of the pipe, making the pipe joint unsuitable for slip lining or pipe rehabilitation applications.
One method for joining profile wall pipes is shown in U.S. Pat. No. 5,362,114 to Levingston. As shown therein, profile wall pipes are joined by threaded engagement. The threads, formed by the helical rib during creation of the pipe, are revealed by shaving or cutting away the inner wall of one pipe section and the outer wall of another pipe section. This allows the two pipe sections to be threaded together. However, joints formed by this method are not sealed and require the use of sealants or gaskets to make the joints liquid tight or leak-proof and, therefore, do not provide the beneficial characteristics of a fusion welded joints. For example, a fusion weld is air tight whereas a threaded joint, even with sealants, is not acceptable for pipelines which require air tight seals.
Thus, methods of joining pipe using prior art collars or inserts result in interior flow obstructions or exterior protrusions which prevent pipe insertion in slip lining applications. Prior art methods utilizing welding rods or mesh require line up equipment and solid wall pipe and, as such, are not acceptable for joining profile wall pipes. Additionally, prior art methods of joining pipe by threading the pipes together do not create leak proof joints without sealants or gaskets. Furthermore, specifications in many applications call for a leak-proof joint having a flush interior and exterior pipe surface that, in trenchless applications, is strong enough to withstand pulling or pushing the pipe through an existing pipeline. The above prior art methods do not satisfy these specifications.
The invention results in a uniform, strong, leak-proof joint with minimal inner or outer obstructions, making it suitable for trenchless, slip lining applications, in addition to direct burial applications. Additionally, the invention requires no fusion machine or special line-up equipment to apply end pressure. The invention may also use an electrical resistance screen element which is an improvement over resistance wires in both the cost and the uniform heat distribution a screen provides. Furthermore, the invention provides a solid profile wall pipe joint or coupling having substantially flush interior and exterior surfaces without the use of a coupler.
Building on the invention, the inventor herein has further improved his early method and apparatus for electrofusion coupling of profile wall thermoplastic pipe. As discussed in the parent, a profile wall pipe comprises an inner wall, an outer wall, and a helical rib joining the two walls together. Due to the thinness of the inner and outer walls, profile wall pipe does not lend itself to the usual bonding methods used for solid wall thermoplastic pipe.
The present invention comprises removing substantially all of either the inner or outer wall, and substantially all of the helical rib, for a predetermined distance from an end of a first pipe to be joined, thereby leaving the first pipe end with only a single wall for a predetermined distance. The wall removed from the first pipe end may be either the inner wall or the outer wall. On the second pipe being joined, a portion of the helical rib is removed for a predetermined distance from an end of the second pipe, leaving the inner and outer walls substantially intact. By removing substantially all of the helical rib adjacent the pipe end, the inner and outer walls adjacent the second pipe end are radially more flexible. This allows the circumference of the inner wall at the second pipe end to be increased or the circumference of the exterior wall at the second pipe end to be decreased, thereby allowing the second pipe end to be coupled to the first pipe end. The amount of the helical rib removed from between the inner and outer walls of the second pipe end can be decreased as the distance from the second pipe end increases, however, such a decrease is not necessary. Additionally, cutting a plurality of longitudinal notches through either the inner or outer wall of the second pipe, extending from the second pipe end, facilitates radial deflection of such a wall.
In a first embodiment, where substantially all of the inner wall and helical rib are removed from the first pipe end, the second pipe end is coupled to the first pipe end by positioning the second pipe end (having a portion of helical rib removed therefrom) into the first pipe end (having substantially only the outer wall remaining) in a manner that provides the coupling with substantially flush interior and exterior surfaces. In a second embodiment of the invention, where substantially all of the outer wall and helical rib are removed from the first pipe, the second pipe end is coupled to the first pipe end by positioning the first pipe end (having substantially only the inner wall remaining) within the second pipe end (having a portion of helical rib removed therefrom), in a manner that provides the coupling with substantially flush interior and exterior surfaces. In the first embodiment, corresponding surfaces of the outer walls of the two pipe ends are mated together to form the coupling. In the second embodiment, corresponding surfaces of the inner walls of the two pipe ends are mated together to form the coupling.
In order to fuse the two pipe ends together, an electrical resistance element can be positioned within the coupling to provide means to fusion weld the pipe ends together. The element is disposed on an appropriate mating surface of one of the pipes, depending on which embodiment of the invention is used, and is energized after joining the pipes together to provide the heat necessary to fusion weld the pipes together to form a weld assembly.
As disclosed in the parent application, the electrical resistance element can comprise a stainless steel screen heating element. The use of a one-piece screen as an electrical resistance element allows complete coverage of the required heating area without the potential to electrically short due to movement of wires or loss of insulation and such screens may be considerably less expensive than other resistance heating methods. Furthermore, electrical shorting can be prevented by separating the element ends radially with a piece of non-conductive material, preferably by using a thin piece of thermoplastic material. The sheet of thermoplastic material is preferably of the same material as the pipes and enhances both the coupling process and the fusion welding of the pipe joint. Although only a portion of thermoplastic sheet positioned between the overlapping ends of the element is needed to prevent a short, using a thermoplastic sheet disposed around the entire length of the electrical resistance element facilitates the coupling of the pipe ends and results in an enhanced bond at the weld assembly.
For example, rather than shaping the two ends of the electrical resistance element to avoid overlap, use of the thermoplastic sheet allows use of an electrical resistance element having a length greater than the circumference of the pipe about which it is to be wrapped. In use, the electrical resistance element is wrapped around the pipe and a thin sheet of polyethylene is placed radially between the opposite ends of the element, thereby insulating the opposite ends and preventing a potential electrical short. When the electrical resistance element is energized, the entire screen element causes the surrounding thermoplastic pipe and polyethylene sheet to heat up, melt, and fuse together. During the fusion process, the thermoplastic sheet, in addition to preventing the screen from shortening, increases the amount of thermoplastic material available for forming the bond.
While a screen heating element can be utilized to fuse the pipes, building on the invention, the inventor has discovered that it is preferable to use one or more thermoplastic coated twisted wire heating elements when joining profile wall pipe without a separate coupler.
While the principal advantages and features of the invention have been described above, a greater understanding of the invention may be attained by referring to the drawings and the description of the embodiments which follow.